Remove ancient "Holiday" storage

Summary:
Ref T11809. This came out of Facebook many years ago for computing the number of business days that revisions had been stale.

We removed the little staleness marker a few months ago and haven't seen complaints about it.

If we did holidays now it would make sense to integrate them more directly with Calendar as real events, but I have no plans to pursue this anytime soon. It's easy enough to add the federal holidays manually (~5 minutes of work per year?) if you want them, and they're commentable/editable and you can add local holidays if you're not in the US.

Test Plan:

• Ran bin/storage upgrade -f.
• Grepped for CalendarHoliday.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11809

Differential Revision: https://secure.phabricator.com/D16788